Access networks connect business and residential subscribers to the central offices of service providers, which in turn are connected to metropolitan area networks (MANs) or wide area networks (WANs). Often, these access networks are hybrid fiber coax (HFC) systems with an optical fiber based feeder network between the central office and a remote node, and an electrical distribution network between the remote node and subscribers. In so-called “Fiber-to-the-x” (FTTx) access networks, the copper-based distribution part of the access network is replaced with optical fiber, e.g., fiber-to-the-curb (FTTC) or fiber-to-the-home (FTTH). In doing so, the capacity of the access network is increased.
Passive optical networks (PONs) are optical networks that can be used in such applications. A PON is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises. A PON typically consists of at least one optical line terminal (OLT) at the service provider's central office and a number of optical network units (ONUs) near end users. A PON configuration can reduce the amount of fiber and central office equipment required compared with point to point architectures. Examples of types of PONs that are defined by international standard bodies include Ethernet Passive Optical Networks (EPONs) and Gigabit Passive Optical Networks (GPONs).
For downstream traffic, the OLT in the PON broadcasts content for all of the ONUs on a single broadcast channel. Addressing is used to identify the appropriate ONU for each packet of the content. However, for upstream traffic, the upstream bandwidth is time shared by all of the ONUs, and only one ONU can transmit data to the OLT at a time. Otherwise, traffic collision between the transmissions of two ONUs will occur. The OLT controls the time sharing of the bandwidth between the ONUs by allocating transmission windows to each ONU, during which each ONU is allowed to transmit upstream traffic. The transmission windows are granted using a time sharing scheme such that only one ONU can transmit to the OLT at a time.
However, if the local clock of one of the ONUs drifts, or if the ONU is failing or malfunctioning, the ONU may begin its transmission window too early or too late and thereby interfere with the transmission of another ONU. This interference may cause a traffic collision, which may result in the loss of data.
Like reference numerals are used in different figures to denote similar elements.